This invention relates generally to inflatable restraint systems and gas generators used to inflate devices such as a vehicle occupant restraint (commonly known as an air bag). More particularly, the invention relates to the autoignition of such gas generators.
It is well known to protect a vehicle occupant using a cushion or bag that is inflated/expanded with gas, e.g., an "air bag" when the vehicle encounters sudden deceleration, such as in a collision. In such systems, the cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the inflatable restraint system, the air bag is commonly inflated in a matter of a few milliseconds with gas produced by a device commonly referred to as "a gas generator" or "an inflator."
Many types of inflator devices have been disclosed in the art for inflating an air bag for use in an inflatable restraint system. One type of inflator device involves the utilization of a quantity of stored compressed gas which is selectively released to inflate the air bag. To properly inflate a typical air bag at an appropriate rate, such a type of device commonly requires the storage of a relatively large volume of gas at relatively high pressures. As a result of the high pressures, the walls of the gas storage chamber are typically relatively thick for increased strength. The combination of large volume and thick walls results in relatively heavy and bulky inflator designs.
Another type of inflator device derives a gas source from a combustible gas generating material, e.g., a pyrotechnic, commonly ignited by means of an igniter having an ignition agent and which upon ignition generates a quantity of gas sufficient to inflate the air bag. Typically, such gas generating materials can produce various undesirable combustion products, including various solid particulate materials. The removal of such solid particulate material, such as by the incorporation of a filtering device within or about the inflator, undesirably increases inflator design and processing complexity and can increase the costs associated therewith.
In addition, the temperature of the gaseous emission of such inflator devices can typically vary between about 500.degree. F. (260.degree. C.) and 1200.degree. F. (649.degree. C.), dependent upon numerous interrelated factors including the desired level of inflator performance, as well as the type and amount of gas generant material used therein, for example. Consequently, air bags used in conjunction with such inflator devices typically are constructed of or coated with a material resistant to such high temperatures. For example, an air bag such as constructed of nylon fabric, in order to resist burn through as a result of exposure to such high temperatures, can be prepared such that the nylon fabric air bag material is coated with neoprene or one or more neoprene coated nylon patches are placed at the locations of the air bag at which the hot gas initially impinges. As will be appreciated, such specially fabricated or prepared air bags typically are more costly to manufacture and produce.
Further, while vehicular inflatable restraint systems are preferably designed to be properly operational over a broad range of conditions, the performance of such inflator device designs can be particularly sensitive to changes in the ambient conditions, especially temperature. For example, operation at very low temperatures, such as temperatures of -40.degree. F. (-40.degree. C.), can affect the performance of various propellants, and thus reduce the air bag pressure resulting from an inflator which contains a fixed available amount of propellant.
In a third type of inflator device, air bag inflating gas results from a combination of stored compressed gas and combustion of a gas generating material, e.g., a pyrotechnic. This type of inflator device is commonly referred to as an augmented gas or hybrid inflator. Hybrid inflators that have been proposed heretofore are subject to certain disadvantages. For example, inflator devices of such design typically result in a gas having a relatively high particulate content.
Various specific inflator devices and assemblies have been proposed in the prior art. U.S. Pat. No. 5,263,740 discloses an assembly wherein within a single chamber is housed both an inflation gas and a first ignitable material, which is subsequently ignited therein.
The housing of both an inflation gas and an ignitable material within a single chamber can result in production and storage difficulties. For example, concentration gradients of such components, both initially and over time as the device awaits actuation, can increase the potential for the release therefrom of ignitable material into the air bag prior to complete ignition, as well as increasing the relative amount of incomplete products of combustion released into the air bag.
Also, gas generators wherein, for example, a fuel and an oxidant are stored in a single chamber, can under certain extreme circumstances be subject to undesired autoignition (i.e., self-ignition) and the consequent dangers that may be associated therewith, both during manufacture and storage.
Further, as the gas mixture resulting from such a single storage chamber assembly will typically be at a relatively high temperature, such designs can be subject to the same or similar shortcomings identified above associated with high temperature emissions.
In an effort to avoid or minimize at least some of these shortcomings, it has been proposed to store the fuel and oxidant in such single chamber gas generators as a fuel lean mixture. However, operation with fuel lean mixtures can itself be subject to various operational difficulties. For example, such a single chamber gas generator operated with a fuel lean mixture can experience ignition difficulties as it can be difficult to ensure that a fuel lean mixture is completely or sufficiently uniformly combustible so as to not unduly hinder performance.
In addition, as a result of the rapid pressure and temperature rises normally associated with inflator devices which house a mixture of oxidant and ignitable material, proper and desired control and operation of such inflator devices can be difficult and/or complicated.
Inflatable restraint systems have been devised for automotive vehicles in which one or more air bags are stored in one or more storage compartments within the vehicle. In general, an air bag provided for the protection of a vehicle driver, e.g., a driver side air bag, is stored within a housing mounted in a storage compartment located in the steering column of the vehicle. Whereas, an air bag for the protection of a front seat passenger, e.g., a passenger side air bag, is typically stored within a housing mounted in the instrument panel/dash board of the vehicle.
In such systems, the gas generators or inflators must be constructed to withstand large thermal and mechanical stresses during the gas generation process. Thus, gas generators have been fabricated using steel for the casing and other structural components, with the structural components commonly joined together by screw threads, roll crimping or welding.
To satisfy light weight specifications, significant weight reduction can be achieved through the utilization of a light metal or material such as aluminum or an aluminum alloy for the generator housing and other structural components. Gas generators made of such materials typically will not experience problems in ordinary use wherein, during the event of a collision, the ignition agent is ignited, followed by the igniting of the gas generant to generate inflation gas. However, the mechanical strength of such lighter weight materials is lowered when overheated to a high temperature.
For example, a problem is encountered when generators utilizing aluminum for the housing construction are subjected to a high temperature environment, such as a bonfire. This problem stems from the fact that at a temperature in the 650.degree. F. (343.degree. C.) range, the pyrotechnics of the gas generator commonly automatically ignite. In this temperature range, the aluminum of the housing structure degrades and tends to rupture or burst, which in turn can result in the projection of pieces and/or fragments in various directions. This problem is not encountered with gas generators that employ steel in the housing structure since steel does not degrade until a much higher temperature of about 1100.degree. F. (593.degree. C.) is reached. Thus, the use of aluminum, in place of steel, in a gas generator, while serving to reduce the weight of the assembly typically results in the gas generator having a lower internal pressure capability. This lower internal pressure capability could be hazardous in a high temperature environment such as the gas generator might be subjected to in the event of a fire whether in storage, in transit, or after installation in a vehicle.
Moreover, it will be understood that regardless the material of fabrication, gas generators can be prone to rupture under certain specific conditions when subjected to sufficiently aggressive reaction of a gas generant material stored therein.
A previously disclosed solution to this problem is the incorporation of an autoignition device in the gas generator. For example, U.S. Pat. No. 4,561,675, Adams et al., assigned to the assignee of the present invention and which patent is incorporated herein in its entirety, discloses an autoignition device that causes the pyrotechnic material in a gas generator to function when the device is subjected to a predetermined high temperature below the ignition temperature of the solid fuel gas generant. The container of the autoignition device is disclosed as being hat shaped and includes a brim and a crown, with the crown attached in thermal contact with the generator housing and with the area of a wall of the container bound by the brim being closed by a foil seal.
The inclusion of an autoignition material in an inflator housing such as is used for inflators for driver side installations is also disclosed in U.S. Pat. Nos. 5,106,119 and 5,114,179 which disclose a housing apparatus wherein, by means of a piece of aluminum foil, a "packet" of autoignition material is held in place in a recess formed in the canister cover. Also, U.S. Pat. No. 5,186,491 discloses the incorporation of an autoignition material within a recess of the gas generator.
In addition, U.S. Pat. Nos. 4,998,751 and 5,109,772, both assigned to the assignee of the present invention and which patents are incorporated herein in their entirety, generally relate to inflator devices. These patents disclose the incorporation, respectively, of "an autoignition device" and "a container" which "holds or contains autoignition granules" in such gas generators within a centrally located recess. Thus, it is known to place autoignition granules within a container within such an elongated gas generator housing at one end thereof, opposite an end of a elongated igniter tube. Furthermore, it is known to use a cup-shaped container to hold such granules.
Unfortunately, the inclusion of an autoignition material in an inflator can be subject to certain drawbacks including those related to increased expense and reduced dependability. First, an autoignition material added to an inflator assembly must typically be carefully prepared, handled and installed, thereby increasing the expense associated therewith. Also, the aging characteristics of typical autoignition materials, whereby the temperature sensitivity of the material may vary over time and may result in inconsistent performance of an aged autoignition material, thereby reducing the dependability associated therewith.